Since advances in semiconductor, laser and power conversion technology are inevitably accompanied by higher powers and higher power densities, such advances cannot be exploited without concomitant advances in thermal management technologies. Thus, thermal management has emerged as a primary limiting design factor in electronic systems.
Personal computers (PCs) provide an excellent example of a thermal management challenge. Namely, with the desktop PCs of today, average heat fluxes experienced can be greater than 150 Watts per square centimeter and in localized regions, often referred to as hot spots, significantly larger average heat fluxes can occur, potentially as high as 500 Watts per square centimeter.
Research on the liquid cooling of electronics has been thriving over the past two decades due to the fundamental limits of the conventional (and ubiquitous) air-cooling approach. For example, for applications such as notebook computers which often require a spatial separation of the heat source and the heat sink, heat pipes are conventionally employed to absorb the heat generated by the heat source, transport it and spread it over the base of a heat sink. However, since heat pipes are passive devices, relying on surface tension to circulate the heat-transfer fluid, there are fundamental limits to the amount of heat that a heat pipe of a given geometry can transport. For example, with many applications, e.g., computer processors and radio frequency power transistors in cellular base stations, the maximum capacity of heat pipes is rapidly being approached or exceeded.
Microchannel cooling is another type of liquid cooling configuration currently under study. Because of the exceptionally high heat transfer coefficients associated with heat transfer to and from the fluid in microchannels, typically greater than or equal to about 1×104 Watts per square meter Kelvin (W/m2K), only a very small temperature difference, e.g., only up to about a five degrees Celsius (° C.), is required to drive heat transfer between the fluid flowing through the microchannels and an adjacent heat source or heat sink. See, for example, R. J. Philips, Microchannel Heat Sinks, 2 Advances in Thermal Modeling of Electronic Components and Systems, 109–184 (1990), the disclosure of which is incorporated by reference herein. The heat transfer coefficient indicates quantitatively how much temperature difference between a surface and a fluid is required to transfer a given heat flux (measured in Watts per square meter) from the surface into the fluid. This has the significant advantage of maintaining the fluid near the operating temperature of the heat source, allowing for a greater temperature difference (driving force) for heat transfer to the ambient environment, which helps minimize heat sink geometry.
One problem with conventional microchannel cooling, however, is that the pressure drop associated with pumping fluid through a microchannel is very high since the channels are so small. As a result, higher power fluid pumps, which are typically larger, heavier, more expensive and more complicated, are needed to overcome the drop. Another problem associated with conventional microchannel cooling is that the efficiency of heat transfer to the fluid remains constant along the length of the microchannel. Namely, the temperature difference between the microchannel wall and the fluid required to transfer a given heat flux into the fluid remains constant along the length of the microchannel. As such, hot spots on the heat source (corresponding to localized regions of high power dissipation) remain at temperatures higher than other regions and introduce thermal stresses from the resulting temperature gradient.
Hot spot mitigation is a formidable problem faced by the electronics industry. See, for example, R. Viswanath et al., Thermal Performance Challenges from Silicon to Systems, INTEL TECH JOURNAL (August 2000), the disclosure of which is incorporated by reference herein. The result is that, increasingly, the hot regions on a die are very localized and limit the power that can be dissipated by the electronics. These limitations further limit the functionality of the die.
Therefore, improved thermal management technologies suitable to accommodate the increasing heat-dissipation needs of the electronics industry are needed.